1. Field of the Invention
This invention relates to a quantum-cryptographic communication system, or in particular to a novel quantum-cryptographic communication system which can realize a many-to-many key delivery in an optical network.
2. Description of the Related Art
In recent years, quantum cryptography has been closely watched and vigorous research and development efforts have been made to promote the quantum cryptography as the next-generation cryptographic technology which may replace the common-key DES (Data Encryption Standard) cryptography and the public-key RSA (Rivest-Shamir-Adleman) cryptography. The information communication employing this quantum cryptography makes it possible for two parties located far from each other to share a secret key without knowledge by third parties.
The quantum-cryptographic communication techniques which have so far been developed, however, are all based on the one-to-one or one-to-many key delivery using a specific fixed line. An attempt to conduct quantum-cryptographic communication in an optical network, therefore, requires the installation of an optical fiber dedicated to each user, thereby constituting an undesirable stumbling block to practical applications of the quantum-cryptographic communication on an optical network.
In order to solve this problem, a method of signal distribution through a beam splitter has been proposed (JP-A-9-502320). In this method, a multiplicity of keys are distributed from the transmitter at random to a multiplicity of users, and therefore the problem is posed that the rate at which the keys are delivered is reduced to 1/N with the increase in the number N of users.
The present invention has been achieved in view of this situation, and the object thereof is to obviate the problem of the prior art and provide a novel quantum-cryptographic communication system for realizing a many-to-many key delivery which allows a given user in an optical network to share a key with another specific user.